The oxidation of alkyl aromatic compounds, e.g., toluene and xylenes are important commercial process. A variety of oxidation products may be obtained including aromatic carboxylic acids such as terephthalic acid (1,4-benzenedicarboxylic acid) and isophthalic acid (1,3-benzenedicarboxylic acid) which are used for example in the polymer industry.
U.S. Pat. No. 2,833,816 discloses processes for oxidizing aromatic compounds to the corresponding aromatic carboxylic acids. A process for the liquid phase oxidation of alkyl aromatic compounds uses molecular oxygen, a metal or metal ions, and bromine or bromide ions in the presence of an acid. The metals may include cobalt and/or manganese. Exemplary acids are lower aliphatic mono carboxylic acids containing 1 to 8 carbon atoms, especially acetic acid.
U.S. Pat. No. 6,355,835 discloses a process for the preparation of benzene dicarboxylic acids by liquid phase oxidation of xylene isomers using oxygen or air by oxidising in the presence of acetic acid as solvent, cobalt salt as catalyst and an initiator. The oxidation step is followed by flashing the said reaction mixture to remove volatile substances and cooling and filtering to get crude benzene di-carboxylic acid as a solid product and filtrate. Recrystallizing the crude benzene di-carboxylic acid to obtain at least 99% purity and recycling of the filtrate are also disclosed.
It is also known in the art that the oxidation products such as aromatic aldehydes, aromatic alcohols, aromatic ketones, and aromatic carboxylic acids may solidify or crystallize at oxidation conditions and/or as the reaction mixture cools. Thus, mixtures of oxidation products may be produced which require further processing to increase the purity of the desired product. For example, in the production of terephthalic acid, the oxidation product is often referred to as crude terephthalic acid as it contains impurities including color bodies and intermediate oxidation products especially 4-carboxybenzaldehyde (4-CBA). To obtain polymer grade or purified terephthalic acid, various purification steps are known in the art including: washing the crude terephthalic acid with water and/or a solvent, additional oxidation or crystallization steps, and reacting a solution of dissolved crude terephthalic acid with hydrogen at hydrogenation conditions usually including a catalyst comprising palladium and carbon. Often several purification steps are used.
U.S. Pat. No. 7,692,036 discloses an optimized process and apparatus for more efficiently and economically carrying out the liquid-phase oxidation of an oxidizable compound. Such liquid-phase oxidation is carried out in a bubble column reactor that provides for a highly efficient reaction at relatively low temperatures. When the oxidized compound is para-xylene and the product from the oxidation reaction is crude terephthalic acid (CTA), such CTA product can be purified and separated by more economical techniques than could be employed if the CTA were formed by a conventional high-temperature oxidation process.
There remains a need in the art for alternate processes that enable the oxidation of alkyl aromatic compounds including oxidation processes that produce aromatic carboxylic acids. Oxidation processes that produce higher purity oxidation products are desirable to eliminate or minimize purification costs. Resulting products with different ratios of contaminant may provide new intermediates useful as raw materials in other applications.